We define the average velocity in a time interval 
as the distance traversed divided by the time it took:
We see that the average velocity is also the slope of the line which connects
the points 
to 
. For the type of motion we
studied here we observe that irrespective of the time interval I pick
I always get the same average velocity. We could confirm this
by explicitly calculating the average velocities in each of the
time intervals corresponding to our little table.
That is for motion with constant
velocity the average velocity is independent of the chosen time interval.
The dimension of velocity is 
and the standard SI unit
is m/s. More common units for velocity are km/h or mph. The conversion
factors are
Thus our cart was traveling at an average velocity of 1.7 mph, just below walking speed.
Talking about speed physicists make a subtle distinction between speed
and velocity. You see velocity is a signed quantity. If I choose to let
x increase to your right but let the cart drive to your left then the
numerator in our equation for the average velocity changes sign and I
get a negative velocity. The negative sign simply tells us that the
cart is moving in the direction opposite to the chosen direction of
increasing x. Speed is an unsigned quatity. it is the magnitude of
the velocity:
